Abstract:

The existence of Ordovician carbonate platform margin has already been proved in Tazhong-Gucheng area, Tarim Basin. However, the distribution, thickness and migration of it are ambiguous because of the great depth of strata, less drilling and limited seismic data quality. Based on analysis of the residual strata, combined with drilling and well logging data, seismic profiles and core samples, the development of Ordovician carbonate platform margin in research area can be chronologically divided into three stages: (1)Lower Ordovician Penglaiba Formation develops an obvious rimmed platform margin, nearly N-S trending, showing “S”-shape distribution; (2)Lower-Middle Ordovician Yingshan Formation generally has similar range of platform margin’s distribution to Penglaiba Formation, but shows reverse “S”-shape around GL1 well; Rimmed and ramped types both exist, and slightly retract inward; (3)Upper Ordovician Qiaerbake Formation-Lianglitage Formation develops two separate platform marginal facies belts in the north and south of Tazhong uplift; the northern one is ramped type; the southern one is rimmed type, with larger width and steeper slope. Generally, in Ordovician period, the platform marginal facies migrated from the overall N-S trending “S”-shape to the westward opening “V”shape. The carbonate platform margins migrated to center of the basin, and the platform edge-slope facies mainly showed a retrogradationalaggradational development in Early-Middle Ordovician. In Late Ordovician, the Tarim basin experienced a significant decline in sea level. Thus, the remnants of platform edge’s distribution underwent a significant narrowing. After the rapid regression, a new round of transgression began, resulting in that the platform marginal facies retreated towards the high part of the Tazhong platform.

Key words: Tazhong-Gucheng area, Ordovician, carbonate platform marginal facies, type, distribution, evolution